The normally right-angled structure between weft and warp, created by the fabric production machinery or similar can often be altered by the machines used for subsequent phases of the processing or treatment. A similar alteration also occurs when working meshed fabrics or the like.
This deformation of the fabric is normally identifiable by the formation of an oblique distortion angle or an arched distortion angle, shown as a1 and a2, respectively, in FIG. 1. In order to bring the distortions back within acceptable standardized limits, it is necessary to use automatic machines, normally defined as “aligners”, which operate according to the measurement of said angles.
Since the fabric is checked in a certain, finite number of points, depending on the number of sensors installed, the determining of these angles is carried out by means of processing the sensor data using mathematical algorithms, such as the calculation of the average values of the local deformations or the analysis of the appropriate functions calculated for the values themselves.
The number of analysis points needed to give a reliable calculation of the deformations may be obtained by installing a sufficient number of sensors, or by a smaller number of sensors, but where each is capable of analysing various points of the fabric by moving the sensor at right angles to the length of the fabric, as illustrated in FIGS. 2 and 3, respectively.
There are already various types of sensors on the market for measuring the distortion angles, made with photodiodes or other devices, which require mechanical parts that rotate or oscillate. There are also static sensors, made with CCD (charge coupled device) linear detectors, connected in various ways in order to form images over time that appear as a succession of lines.
However, all these solutions function only if there is a relative movement between the fabric and the sensor, as described, for example, in patent EP 0 741 290. Furthermore, the linear structure requires the sensor to be mounted in a pre-set direction and the measurement solution is intrinsically different in the two right-angled directions.
Therefore, generally speaking, almost all the existing systems include the use of one or more light sources in continuous operation, which requires a certain attention to energy dissipation.